Mechanical seals generate significant heat loads at an interface between a flat rotating seal face and an opposing flat stationary seal face. These heat loads are due to contact friction between the faces and viscous shear of the lubricating fluid between the faces. Removal of this heat is typically through convection with the gas or liquid fluids at the surfaces of the rotating and stationary seal faces, and conduction at locations where the face comes into contact with metal parts.
The temperature at the seal faces varies directly with the heat generation, and varies inversely with the convection and conduction coefficients. In fluids that act as poor lubricants and as poor convective media such as air or other gases, conduction of heat into the metal components of the seal and the sealed equipment becomes critical in determining the seal face temperature.
In typical mechanical seals, the conduction coefficient is poor between a seal face and a metal part due to the rigid nature of both the mechanical seal face and the metal part, resulting in air gaps between the parts. In dry running conditions, the continuing contact between the seal faces can lead to excessively high seal face temperatures wherein the excessive temperature can rapidly lead to seal failure such as by damaging the carbon seal rings and damaging O-rings, particularly those in contact with the other ring which may be made of silicon carbide.
With respect to such problems, particularly under dry running conditions, some mechanical seals may effect heat removal from faces, wherein, for example, the surface area of the seal faces in contact with the process fluid can increase convection of heat. This works well under normal operating conditions where convection is the primary mode of heat removal, however this does not solve the problem of high seal face temperatures associated with dry running conditions. Further, a seal face surface in direct contact with a metal surface tends to conduct heat from the seal ring to the metal surface. This improves conductivity, but only to the extent that the flatness of the metal and seal face surface at the contact interface is controlled. Lapping can improve the surface contact at the interface although this has the trade-off of significant additional expense.
It therefore is an object of the invention to overcome difficulties with dissipating heat resulting from dry running and other upset conditions.
In view of the foregoing, the invention relates to a method for enhancing heat removal from mechanical seal faces which results in a significant increase in the ability of the seal to tolerate poor lubrication conditions such as dry running where heat loads are increased and convection cooling is poor.
The mechanical seal of the invention employs the following features: (1) A thin, flat sheet of flexible thermally conductive graphite material placed between one of the mechanical seal faces and a metal surface such as a metal surface defined by a shaft sleeve or gland serving as a seal ring holder. (2) The graphite sheet is located axially between the more thermally conductive mechanical seal face and the metal face holder part. The conductive seal face material preferably is a ceramic or cermet material such as silicon carbide, tungsten carbide, silicon nitride, aluminum oxide, or a metallic material such as stainless steel. (3) Fluid pressure and spring forces are used to create a compressive load between the seal face, graphite sheet, and metal part to maximize continuous contact between the opposed faces of these parts and the opposite sides of the graphite sheet. (4) The graphite sheet preferably has a thickness of 0.005″ to 0.030″.
More particularly, the invention incorporates a thin sheet of graphite sheet material that is sandwiched between a silicon carbide, tungsten carbide, silicon nitride, or aluminum oxide seal face material and a metal component of the seal such as the sleeve or gland. The sheet is housed or sandwiched axially between the surface of the seal face, which is located opposite of the primary sealing interface defined between opposed seal faces, and the surface of either a rotating or stationary metal component such as a sleeve, gland, or rotating face support.
The sheet material is used to enhance the conduction of heat from the seal face into the metal component, thereby reducing the seal face temperature and improving seal performance, especially in poor lubrication conditions such as dry running.
In dry run tests conducted on a prototype mechanical seal design using a conventional direct metal conduction path from the mechanical seal face to a metal seal ring holder, the seal face temperature would reach excessive temperatures in under 10 minutes of dry running that were sufficiently elevated to damage the seals and cause seal failure. This temperature would degrade the seal face materials and elastomers in contact with the seal faces, rapidly resulting in seal failure. When the inventive seal was tested with the graphite sheet between the seal face and the metal sleeve, seal face temperatures were substantially less and did not reach the level which would cause seal failure. The seals ability to operate without seal failure is extended several times longer and in some cases, may be able to avoid seal failure from elevated temperatures for as long as one hour of dry running.
The graphite sheet material comprises a commercially available industrial grade flexible graphite flat sheet material, and has been shown to be effective in thicknesses from 0.005″ to 0.030″ in testing. Significant features of the sheet that enable conduction are: (1) the ability of the material to conform to the surface variations of both the seal face and metal part, increasing the contact between the parts and therefore the conductivity; (2) high thermal conductivity in the transverse plane of the sheet, which enables improved conduction in any areas where the sheet does not fully conform; and (3) high thermal conductivity in the axial plane of the sheet, which enables heat flow.
The general use of a gasket between a seal face and a metal component is provided in some seals. However, in these applications, the gasket material is not thermally conductive and only serves as a means to prevent damage or distortion to the seal faces.
Further, some commercially available seals use a corrugated graphite gasket between a carbon graphite seal face and a metal component. In this type of application, the use of such a gasket is as a compliant seat and drive mechanism for the face. In this instance, the gasket is a thermally conductive graphite material, but the carbon graphite seal face material is not thermally conductive and thus does not provide the operational benefit of heat removal from the interface defined between two opposed seal faces. Hence, the carbon graphite seal impedes transfer of heat from the other seal ring.
The improved mechanical seal of the invention thereby enhances conduction of heat away from seal faces through the use of the flat graphite sheet wherein this feature can be incorporated into development of new mechanical seal products targeted toward chemical and general industrial applications worldwide. For this product alone, the performance increase caused by the sheet is significant in that it enhances the ability of the seal to survive and recover when encountering off design operation conditions that typically cause seal failure with existing seals. This improved heat transfer capability results in increased reliability and overall product life for the seals.
In addition to new products, the graphite sheet may also be incorporated into existing products and other new developments for performance enhancements of existing seal products.
Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings.
Certain terminology will be used in the following description for convenience and reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the arrangement and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import.